In recent years, there has been demand for further improvement of automobile fuel efficiency and exhaust gas purification from a standpoint of environmental protection. Consequently, adoption of exhaust heat recovery units and EGR (Exhaust Gas Recirculation) coolers in automobiles continues to increase.
An exhaust heat recovery unit is an apparatus that improves fuel efficiency by, for example, using heat from engine coolant for automobile heating and using heat from exhaust gas to warm up engine coolant in order to shorten warming-up time when the engine is started up. The exhaust heat recovery unit is normally located between a catalytic converter and a muffler, and includes a heat exchanger part formed by a combination of pipes, plates, fins, side plates, and so forth, and entry and exit pipe parts. Usually, fins, plates, and the like have a small sheet thickness (about 0.1 mm to 0.5 mm) to reduce back pressure resistance, and side plates, pipes, and the like have a large sheet thickness (about 0.8 mm to 1.5 mm) to ensure strength. Exhaust gas enters the heat exchanger part through the entry pipe, transfers its heat to a coolant via a heat-transfer surface such as a fin, and is discharged from the exit pipe. Bonding and assembly of plates, fins, and so forth forming the heat exchanger part of an exhaust heat recovery unit such as explained above is mainly carried out by brazing using a Ni-containing brazing metal.
An EGR cooler includes a pipe for intake of exhaust gas from an exhaust manifold or the like, a pipe for returning the exhaust gas to a gas intake-side of an engine, and a heat exchanger for cooling the exhaust gas. The EGR cooler more specifically has a structure in which a heat exchanger including both a water flow passage and an exhaust gas flow passage is located on a path along which exhaust gas is returned to the gas intake-side of the engine from the exhaust manifold. Through the structure described above, high-temperature exhaust gas at the exhaust-side is cooled by the heat exchanger and the cooled exhaust gas is returned to the gas intake-side such as to lower the combustion temperature of the engine. Accordingly, this structure forms a system for inhibiting NOx production, which tends to occur at high temperatures. The heat exchanger part of the EGR cooler is made by overlapping thin fins and plates, for reductions in weight, size, cost, etc. Bonding and assembly of these thin plates is mainly carried out by brazing using a Ni-containing brazing metal.
Since bonding and assembly for a heat exchanger part in an exhaust heat recovery unit or an EGR cooler such as described above are carried out by brazing using a Ni-containing brazing metal, materials used in the heat exchanger part are expected to have good brazing properties with respect to the Ni-containing brazing metal. Moreover, a heat exchanger part such as described above is expected to be highly resistant to oxidation caused by high-temperature exhaust gas passing through the heat exchanger part. The exhaust gas includes small amounts of nitrogen oxides (NOx), sulfur oxides (SOx), and hydrocarbons (HC) that may condense in the heat exchanger to form a strongly acidic and corrosive condensate. Therefore, materials used in a heat exchanger part such as described above are expected to have corrosion resistance at normal temperatures. In particular, because brazing heat treatment is carried out at high temperature, it is necessary to prevent formation of a Cr depletion layer due to preferential reaction of Cr at grain boundaries with C and N, which is referred to as sensitization, in order to ensure that corrosion resistance is obtained.
For the reason described above, heat exchanger parts of exhaust heat recovery units and EGR coolers are normally made using an austenitic stainless steel such as SUS316L or SUS304L that has a reduced carbon content and is resistant to sensitization. However, austenitic stainless steels suffer from problems such as high cost due to having high Ni content, and also poor fatigue properties and poor thermal fatigue properties at high temperatures due to its large thermal expansion when used in an environment in which constraining force is received at high temperature and with severe vibration, such as when used as a component located peripherally to an exhaust manifold.
Therefore, steels other than austenitic stainless steels are being considered for use in heat exchanger parts of exhaust heat recovery units and EGR coolers.
For example, PTL 1 discloses, as a heat exchanger component of an exhaust heat recovery unit, a ferritic stainless steel in which Mo, Ti, or Nb are added and Si and Al content is reduced. PTL 1 discloses that addition of Ti or Nb prevents sensitization by stabilizing C and N in the steel as carbonitrides of Ti and Nb and that reduction of Si and Al content improves brazing properties.
PTL 2 discloses, as a component for a heat exchanger of an exhaust heat recovery unit, a ferritic stainless steel having excellent condensate corrosion resistance in which Mo content is defined by Cr content, and Ti and Nb content is defined by C and N content.
Furthermore, PTL 3 discloses, as a material for an EGR cooler, a ferritic stainless steel in which added amounts of components such as Cr, Cu, Al, and Ti satisfy a certain relationship.
Additionally, PTL 4 and 5 disclose, as a component of an EGR cooler and a material for a heat exchanger part of an EGR cooler, a ferritic stainless steel containing 0.3 mass % to 0.8 mass % of Nb and a ferritic stainless steel containing 0.2 mass % to 0.8 mass % of Nb.